RAMAN SPECTROSCOPY AND ATOMIC FORCE MICROSCOPY OF SOOT SAMPLED IN HIGHPRESSURE DIFFUSION FLAMES

The objective of the present work is to assess the effect of pressure on nanostructure, morphology and dimension of soot particles from a set of co-flow methane/air laminar diffusion flames stabilized in a high-pressure combustion chamber. To this aim, particles produced in a pressure range between 5 and 20 bar have been collected by thermophoresis and characterized by means of Raman spectroscopy and Atomic Force Microscopy (AFM). First-order Raman spectra have been acquired at different heights above the burner for each pressure. No significant changes in the nanostructure and composition of soot particles were detected as a function of pressure, while nanostructure and composition have been observed to change with particle residence time within flame. From AFM measurements, Particle Size Distribution Functions (PSDs) have been obtained through a dimensional statistical analysis. PSDs have been found to be mostly bimodal, with a first particle mode below 10 nm slightly decreasing with pressure and a second particle mode between 10 and 20 nm rather constant with pressure. Moreover, a comparison between PSDs from AFM and PSDs from Transmission Electron Microscopy reported previously showed a good complementarity between those two techniques. This study represents a first attempt to characterize and correlate the structural and morphological properties of soot particles produced at high-pressures, which are the typical operating conditions of most practical combustion devices.